Updating Dockerfile baseimages to mach ocp-build-data config

This PR is autogenerated by the [ocp-build-data-enforcer][1].
It updates the base images in the Dockerfile used for promotion in order to ensure it
matches the configuration in the [ocp-build-data repository][2] used
for producing release artifacts.

Instead of merging this PR you can also create an alternate PR that includes the changes found here.

If you believe the content of this PR is incorrect, please contact the dptp team in
#aos-art.

[1]: https://github.com/openshift/ci-tools/tree/master/cmd/ocp-build-data-enforcer
[2]: https://github.com/openshift/ocp-build-data/tree/openshift-4.6/images

Author: openshift-bot

Dockerfile

@@ -1,4 +1,4 @@
-FROM registry.svc.ci.openshift.org/ocp/builder:rhel-8-golang-openshift-4.6 as builder
+FROM registry.svc.ci.openshift.org/ocp/builder:rhel-8-golang-1.15-openshift-4.6 as builder
 
 ENV GO111MODULE auto
 ENV GOPATH /go